The present invention relates to a process for operating a blast furnace wherein iron ore is reduced to form pig iron, where reducing gas superheated to a temperature of up to 2000.degree. C. and higher is injected into the lower part of this furnace, for example at the level of the main blast furnace tuyeres.
Present-day energy saving considerations have pushed the industrial sector, in particular iron and steelmakers, to reduce consumption of primary energy to a strict minimum and to replace certain types of energy by others that are cheaper or more readily available. As long as hydrocarbons could be purchased in large amounts and at a very low price tuyere injection of oil, natural gas, etc., made possible to replace as much as 20% of the metallurgical coke in the blast furnace. Under the present conditions, these types of injectants have to be replaced by other fuels such as, for example, coal.
Another possibility which is a well known process is the injection of hot reducing gas at the level of the main tuyeres of the furnace in order to reduce coke consumption. This reducing gas contains primarily CO, H.sub.2 and possibly N.sub.2, as well as small amounts of CO.sub.2 and H.sub.2 O. It can be produced outside the furnace, in an independent unit or preferably directly in the injection circuit of the furnace. Such reducing gas can be injected into the furnace, to replace, totally or in part, the hot blast normally used. However, it must be well understood that within the scope of the present invention, those tuyeres through which hot reducing gas is injected are not used for blowing hot blast or any similar oxidizing agent. In the advantageous embodiment wherein hot reducing gas is injected through all tuyeres, this hot reducing gas replaces totally the blast normally used in conventional operation. In another embodiment hot reducing gas may be injected through some tuyeres only and hot oxidizing gas (air, oxygenated air, and so on) is injected through the remaining tuyeres.
Different methods and apparatuses are proposed, by the present applicant and others, for producing the reducing gas from different fuel (solid, liquid or gas) and oxidizing agents, including the use of recirculation gas as described in Canadian Pat. No. 1,007,050.
The high temperature to which this gas is brought, about 2000.degree. C., can be obtained by different ways, preferably by means of electric devices such as plasma furnaces, arc heaters or similar equipment which have the double advantage of facilitating the necessary chemical reactions to produce such gas and furnishing the heat required for furnace operation. Such a process has been claimed by the applicant in U.K. Pat. Nos. 1,335,247; 1,332,531; 1,354,642; 1,459,659; and 1,488,976. Intensive research has been carried out to confirm the possibility of applying such a process on blast furnace equipment with a highly efficient operation for making hot metal.
This research was based on previous observation that heat and mass transfer in the blast furnace process is in no way modified if, instead of creating metallurgical reactions by the traditional method where gas is produced within the furnace by the combustion of coke with the hot blast, a gas having substantially the same composition and temperature is injected, such gas having been produced either in the injection circuit or outside the furnace and injected through the same tuyeres.
During the research, it was noted that other types of hot reducing gas can be injected. In this case, the blast furnace is operated in a significantly different manner from that of traditional blast furnace operation.
A paper presented by the inventors in Detroit, at the Iron-making Conference of Iron and Steel Society of the American Institute of Mining Metallurgical and Petroleum Engineers, March 1979, and published in the Proceedings, contains indications in this field. One of the main differences between the conventional blast furnace operation and the injection of superhot reducing gas process, as discussed in said paper, is the very low coke rate achieved. The lowest coke rate obtained during these tests described in said paper was 179 kg of dry coke/mtHM (metric ton of hot metal). This coke rate is substantially less than the value of 717 kg of dry coke/mtHM obtained in the experimental blast furnace when operated in a conventional manner (see Table I).
TABLE I ______________________________________ Experimental Furnace Injection of Conventional Super hot re- Conventional versus Blast Furnace ducing Gas Invention Operation Operation ______________________________________ Blast Quantity (Nm.sup.3 /mtHM) 2070 0 Temperature (.degree.C.) 748 0 Reducing H.sub.2 O + CO.sub.2 (%) 0 6.9 Gas Quantity (Nm.sup.3 /mtHM) 0 2800 Temperature (.degree.C.) -- 2070 Coke Kg/mtHM 717 179 Rate Pig Iron Productivity (mtHM/d) 1.29 1.35 S. (%) 0.64 0.31 Temperature (.degree.C.) 1410 1360 Top Gas Temperature (.degree.C.) 145 * ______________________________________ The temperature of the top gas is not known because it was so high that cooling water had to be added to the furnace top to prevent damage to the furnace.
During the research, it also appeared that the amount of hot reducing gas consumed to achieve these results was far in excess of what is theoretically required. This leads to an excessive energy consumption and is detrimental to the economics of the process. Furthermore, it was not then possible to adjust the furnace productivity to the selected set point.
Having thus proven that the application of this new technique to a conventional blast furnace is possible, new trials were conducted to find the best operating conditions which led to the present invention.
However, on the basis of the test results obtained from the new trials, it becomes possible to develop a method of control which simultaneously meets all the targets aimed at (i.e., coke rate, iron quality, furnace productivity and minimum energy consumption) and which also shows supplementary advantages compared to the conventional furnace operation.